An astronaut in a returning space vehicle is riding a bomb. With a re-entry speed just below five miles per second, an inert mass like a space capsule has energy of motion seven times as high as the explosive power of the same mass of TNT. If something goes awry, a phenomenal explosion is inevitable. Unfortunately, something did go wrong with flight STS-107.

Let us leave the technical aspects aside, and consider instead the human angle. Astronauts know the risks, and face them courageously and willingly. Although the advent of space tourism may have led the public into thinking that spaceflight is now safe, in fact for the foreseeable future all spacecraft will be experimental vehicles.

The expected failure rate for shuttle missions is around one in 100, maybe more. Everyone involved knows that. Even a decade ago commentators were arguing that the International Space Station could not be built to plan unless Nasa's fleet is augmented, because it is to be anticipated that some shuttles will be lost along the way.

Despite this, there is no shortage of capable volunteers. The astronaut corps has dozens of trained members and there are crews, and multiple back-ups, ready and willing to man the next launch. The hiatus now is for one reason only: to assure the American public that everything possible is done to maximise the safety of the remaining shuttles, and the human cargo they carry.

Should our concern for their safety lead to an end of manned spaceflight? Clearly, no. Should we stop people from climbing mountains, or sailing the oceans? Should we cry a halt to motor racing? Not so long as the people involved do not recklessly endanger the lives of others.

S pace research is expensive. But it can justify the cost 1,000 times over. Much of Britain being brought to a standstill last week by a smattering of snow was shameful because, unlike a couple of decades ago, we knew precisely when and where it was coming, from our meteorological satellites. When I was a child it took four days for the film of the first Cassius Clay versus Sonny Liston title fight to arrive and be shown on British TV, whereas now we have instantaneous viewing of the Superbowl from San Diego, and the cricket from Australia - although not everyone will agree this is a good thing.

Nowadays many space activities may be carried out by robotic craft, controlled from the ground or by their on-board computers. But there is a limit to what can be done remotely, or using artificial intelligence. Space agencies try to minimise cost in every way, and anything involving manned flight implies far higher expenditure, but in the end there is no replacement for a human brain. Many probes have been sent to Mars, and this year Nasa and the European Space Agency will launch others, but these have all been robotic craft with limited capabilities. To understand Mars, and conduct a proper search for life, eventually we'll need to send a geologist with a rock hammer - plus, of course, some pretty sophisticated analysis equipment.

Moreover, if Nasa decided to abandon all manned missions, its budget would be cut. American voters like to view pictures of distant planets returned by robotic satellites, but even more they appreciate seeing astronauts in space suits bearing the Stars and Stripes.

All great human projects cost lives. Build a bridge, and workers are killed in construction accidents. The human cost of the loss of Columbia is not only the seven astronauts, but also those who will die in building its replacement, either working directly on its fabrication, or those labouring to pay the tax dollars that will fund it. It was ever thus: check out how many of Captain Cook's sailors died on his voyages, despite his success in countering scurvy.

"Do not go gentle into that good night," exhorted Dylan Thomas of us all. The courageous astronauts of STS-107 did not, and set an example to follow, in whatever walk of life. To some the inspiration will be pivotal: how many Indian schoolgirls will have their life courses changed by learning of the achievements of Kalpana Chawla?

Ultimately, like all explorers, astronauts lead their lives in a way described by Tennyson more than 160 years ago: "To strive, to seek, to find, and not to yield." We owe it to those who have died, and those who yet will perish in helping to conquer space for all mankind, not to yield in our pursuit of what lies above and beyond this island earth.

· Duncan Steel is reader in space technology at the University of Salford. He has worked for both Nasa and the ESA.